Ukraine’s Border Guard Service on Monday accused Russia of effectively blockading the Kerch Strait by subjecting vessels bound for Ukrainian ports to an extremely slow approval process that allows only a few ships to pass per day.

The U.S. Navy deployed its guided-missile destroyer USS McCampbell near Russia-contested waters in the Sea of Japan on Wednesday as it begun taking the necessary steps to sail a warship into the Black Sea, west of Russia, amid increased tensions in the wake of the Kremlin’s seizure of three Ukrainian military vessels.

Russia is holding naval war exercises and surface-to-air missile drills in the Black Sea region amid tensions with Ukraine and a U.S.-led alliance over the capture of three Ukrainian naval vessels and their crew off the coast of Moscow-annexed Crimea last month, the Kremlin’s defense ministry reportedly announced on Tuesday.

The Border Guard Service of Ukraine announced a ban on Friday on male Russian nationals between the ages of 16 and 60 entering the country for the duration of martial law, currently scheduled to last 30 days.

The naval conflict this weekend between Russia and Ukraine was not a sudden development. Pressure has been building between the two nations for a long time, arguably even before Ukraine broke away from the disintegrating Soviet Union in 1991.

Russia alarmed the world on Sunday by reportedly seizing Ukrainian vessels in the Sea of Azov, attacking Ukrainian assets and blocking some of its most important port cities through its illegal command of the Crimean peninsula.

An 18-year-old allegedly massacred his fellow students with a shotgun on Wednesday at a vocational school in Crimea, the Ukrainian territory Russia annexed in 2014, killing 19 and injuring more than 50 others before committing suicide.

The Russian government reacted with outrage on Wednesday following a declaration from the State Department confirming that Washington would never legitimize Moscow’s illegal invasion and annexation of Crimea, Ukraine.

Russian President Vladimir Putin warned Western powers on Thursday against deepening military alliances with Georgia and Ukraine, asserting that Russia would “respond proportionately” to “aggressive moves” against it.

U.S. President Donald J. Trump went right to work in Brussels at the NATO summit Wednesday morning, taking truculent Germany to task for coming to the United States for defence, while simultaneously paying billions of dollars to the Russian Federation for energy.

“Well you know, you have to ask President Obama because he was the one that let Crimea get away, that was during his administration,” Trump said during a press conference with reporters at the G7 Summit in Canada.

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Obama’s complaint about the “19th century” echoes a similar complaint made by his administration. In 2014, then-Secretary of State John Kerry complained that Russian President Vladimir Putin was behaving in a “19th century fashion” rather than obeying the niceties of 21st century diplomacy when he invaded Crimea.